The technical field of this invention concerns analytical devices and methods for determining the presence of a bindable target substance (i.e. analyte) in a liquid sample possibly containing such substance. Various types of analytical devices, and methods employing the devices, have been used for this purpose. Many of these devices employ reaction membranes onto which a receptor capable of specifically binding to the target substance, is immobilized. In the assay that employs these types of devices, typically the sample to be tested is applied to the reaction membrane. If target substance is present in the sample, it binds to the immobilized receptor. Various methods are used to determine whether the target substance has bound to the receptor, thus indicating its presence in the sample. For immunoassays, where the target substance is an antigen, it is common to use antibodies that are capable of specifically binding to the antigen and that are labeled with detectable markers. When the labeled antibody is added to the reaction membrane, it will bind to the target antigen, if present, and the marker (e.g. fluorescent label, colored reagent, detectable enzyme marker, etc.) is detected.
Membrane-based analytical assays and devices have greatly simplified medical diagnostics. The results of a membrane-based analytical assay can be obtained in a matter of minutes. Quantitative results can be provided by special instruments designed to read the test results. Various types of membrane-based analytical assays are described in U.S. Pat. No. 5,006,464 to Chu et al., U.S. Pat. No. 4,818,677 to Hay-Kaufman et al., and U.S. Pat. No. 4,632,901 to Valkirs et. al., and U.S. Pat. No. 5,185,127 to Vonk et al. There is a continued need to further simplify membrane-based analytical assays and to provide more rapid and more sensitive assays.